It is known that panel bender machines are used in the industry for the manufacture of metal sheet products which allow a succession of bends to be made on the same metal sheet, in such a way as to obtain a finished product such as, for example, the hood of a cooker or the shelf of a stand.
It is also known that the panel bender machines or metal sheet benders normally consist of:                a fixed workbench for supporting the material, for example a metal sheet, to be bent;        a supporting frame for a clamping press;        a punch or presser, forming part of the press, and a corresponding counterpunch or counterblade, being designed to clamp the material during the bending process;        one or more auxiliary elements, inserted manually or automatically, commonly called auxiliary tools, forming part of the press, to be positioned between the punch and the corresponding counterpunch or counterblade, made specially and designed to clamp the material during the bending process of special parts;        one or more bending blades which can move during use towards the material being processed;        suitable mechanisms for moving the bending blade or blades along the workbench for shaping the part clamped between the punch and the counterpunch;        suitable mechanisms for moving the punch which allow the clamping and release of the metal sheet, also in the presence of auxiliary tools, guaranteeing a pressing force always corresponding to the length and thickness to be bent;        means for feeding the metal sheet, or profile, towards the blades during the work cycle;        transducers and sensors of various types for process control, connected to an electronic control unit designed to control the production process.        
A panel bender machine of the known type described above, marketed by the Applicant, comprises a C-shaped blade holder structure, which is able to move according to two directions reciprocally at right angles to the fixed workbench, on which the bending blade or blades is/are fixed.
The profile of the bend obtainable with a prior art panel bender is not only the typical 90° profile obtainable with a manual bender. The simultaneous control of the positioning of the metal sheet and the pressure exerted on it makes it possible to obtained radiused profiles.
The use of conventional blades, special tools and dies, the intervention of which is likewise inserted in the bending cycle, makes it also possible to obtain special profiles, without the need for intervention of the operator with changes to the length or the special tool used.
The blades, according to the conventional construction concept, are, as stated above, supported by a C-shaped load-bearing structure mounted on the main frame and the unit comprises two blades: the upper one for making negative bends (downwards), and the lower one for positive bends (upwards).
The system controls the size of the angles and the thickness of the metal sheet, adjusting the position of the blades using electric motors. All the movements are also executed by electric motors. A special mechanism guarantees the parallelism of the bending unit movements.
The upper punch or presser element is sectional for obtaining the size adequate for the part to be processed and contractable so as to allow the extraction of the part processed. It is mounted on an electro-welded structure with four arms hinged in the rear part of the main frame.
Thanks to the action of suitable mechanisms each segment may be released and repositioned extremely easily, since a trigger prevents the falling from the tool holder bar.
The movements of the C-shaped structure and of the upper tool are caused by hydraulic cylinders or by electric motors.
The cylinders, or the electric motors, are controlled in position by a suitable system (numerical control or other system) so as to allow the highest precision during all the bending phases.
In the conventional hydraulic panel bender machines, as in other panel bender machines present on the market, there is a kinematic structure which causes and controls the movement of the blade holder unit.
This structure may in some cases be of the pentalateral type, that is, consisting of a closed kinematic chain with five members connected by five kinematic pairs.
In the hydraulic machines, the conventional kinematic chain of the pentalateral type is, however, used for providing twisting rigidity to the machine and, therefore, not with specific mechanical functions.
With patent application PCT/IT2004/000581 the same Applicant has described a particular kinematic chain with two degrees of freedom, allowing the electrical actuation of the bending blades.
The same Applicant has also introduced onto the market a series of panel bender machines characterised by the electrical actuation of the bending axes and control of the punch/presser, that is, of all the axes which deliver torques and absorb significant outputs exploiting the above-mentioned invention.
This series of machines has the following features:                reduced energy consumption (less than half that of a corresponding hydraulic machine);        quieter and greater environmental protection;        better control of the pressing and bending axes with consequent improved results in terms of finish of the component;        better performance in terms of speed and cycle times compared with that of the machines on the market.        
Patent document EP-A-1410855 describes a bender machine which, using a TV camera, a display and suitable software, allows measurement of a real bending angle, comparison with a nominal bending angle, calculation of the angle of deviation between the real angle and the nominal angle, and calculation of a new real bending angle which, taking into account the deviation, makes the real bending angle coincide with the nominal bending angle as far as possible.
More specifically, the procedure described above is performed by representing the first real angle on the display, positioning on the display a first reference indicator, calculating the angle of deviation and fixing a new bending angle which takes into account the above-mentioned angle of deviation.
The above-mentioned operations are specifically performed on the display by a skilled operator who, once the image has been taken by the TV camera of the first bend made, which is shown on the display, overlaps on the display a line representing the nominal bending angle of the metal sheet, calculates the angle of deviation and enters the new bending angle in the work program in order to obtain a metal sheet bent at an angle as close as possible to the nominal bending angle.
It may be noted that the system in the above-mentioned patent application does not execute any processing of the images, and does not obtain from them any additional information, and limits itself to showing them on the display and drawing on them a line corresponding to the desired bending angle.
On the other hand, the system in the above-mentioned patent application requires the intervention of an operator who, on the basis of his/her professionalism and experience, assesses the angular differences on the display and enters in the system the most suitable angular corrections.
For this reason, the system appears confused, laborious, slow and susceptible to errors in the manual procedure at the basis of its operation.